This invention relates generally to semiconductor integrated circuit techniques, and more particularly to a technique which is effectively utilized for LSI (large Scale Integrated Circuits) for controlling peripheral units of a microcomputer, such as a CRT (cathode ray tube) controller.
CRT controllers, DMA (direct memory access) controllers, disc controllers, and the like, are known as controller LSIs for controlling peripheral units constituting a microcomputer system, such as CRT displays, floppy disks, and so forth. In order to detect the content of an internal register or status data in such a controller LSI, it has been necessary in a system using the conventional controller LSI to first examine the status of the LSI by means of a microprocessor or to confirm whether or not the LSI is under the accessible state where it is accessible y the microprocessor, and then to read the register content or the status data in that LSI by means of the microprocessor. Therefore, a waiting state occurs in the system and hence bus efficiency drops.
FIG. 4 of the accompanying drawings is a block diagram of a system using a conventional controller LSI. When it is desired to use the content of an internal register or an internal signal for control outside of a controller LSI 10, a sub-control circuit 13' which latches desired data or signals and generates a control signal for control equipment 15 on the basis of the latched content, as well as a timing generation circuit 14 which decides the latch timing at the sub-control circuit 13' and data output timing for the microprocessor and other devices, have been necessary. In this case, a problem develops in that external circuits of the controller LSI become complicated in construction.
For details of the CRT controller and the DMA controller, refer to the following literatures, respectively:
"Nikkei Electronics" May 21, 1984, No. 343, pp. 222-254 published by Nikkei-McGraw Hill Co. PA1 "Nikkei Electronics" Aug. 2, 1982, No. 296, pp 129-158 published by Nikkei-McGraw Hill Co.